1. Field of the Invention
This invention relates generally to tools and methods for drilling an inclined borehole using rotary drilling techniques, and particularly to rotary directional drilling tools and methods where the axis of rotation of the drill bit is articulated relative to the longitudinal axis of the lower end portion of the drill string in a manner which allows the bit to drill a steered, directional borehole in response to drill string rotation.
2. Description of the Related Art
An oil or gas well often has a subsurface section that is drilled directionally, that is a portion of the wellbore is inclined at an angle with respect to vertical and with the inclination having a particular compass heading or azimuth. Although wells having deviated sections may be drilled most anywhere, a large number of such wells are drilled offshore from a single production platform in a manner such that the bottoms of the boreholes are distributed over a large area of a producing horizon over which the platform is centrally located.
A typical procedure tier drilling a directional borehole is to remove the drill string and bit by which the initial, vertical section of the well was drilled using conventional rotary techniques, and run in a mud motor having a bent housing at the lower end of the drill string which drives the bit in response to circulation of drilling fluids. The bent housing provides a bend angle such that the axis below the bend point, which corresponds to the rotation axis of the bit, has a "toolface" angle with respect to a reference, as viewed from above. The toolface angle, or simply "toolface ", establishes the azimuth or compass heading at which the borehole will be drilled as the mud motor is operated. Once the toolface has been established by slowly rotating the drill string and observing the output of various orientation devices, the motor and bit are lowered to bottom and the mud pumps are started to cause the bit to be turned. The presence of the bend angle causes the bit to drill on a curve until a desired inclination has been built up. Then the drill string is rotated at the surface so that its rotation is superposed over that of the mud motor output shaft, which causes the bend point to merely orbit around the axis of the borehole so that the bit drills straight ahead at whatever inclination and azimuth have been established. If desired, the same directional drilling techniques can be used near total depth to curve the borehole back to the vertical and then extend it vertically down into or through the production zone. Measurement-while-drilling (MWD) systems commonly are included in the drill string above the motor to monitor the progress of the drilling so that corrective measures can be instituted if the various borehole parameters are not as planned.
However, when drilling is being done with a mud motor and the drill string is not being rotated, various problems can arise. The reactive torque due to operation of the motor and bit can cause the toolface to gradually change so that the borehole is not being deepened at the desired azimuth. If not corrected the wellbore may extend to a point that is too close to another wellbore, and be considerably longer than necessary. This of course will increase drilling costs substantially and reduce drainage efficiency. Moreover, a non-rotating drill string may cause increased frictional drag so that there is less control over weight-on-bit, and its rate of penetration, which also can result in substantially increased drilling costs. Of course a nonrotating drill string is more likely to get stuck in the wellbore than a rotating one, particularly where the string extends past a permeable zone where mud cake has built up.
A patent which is related to the field of this invention is U.S. Pat. No. 5,113,953, Noble, which proposes contra-rotating the drill bit axis at a speed that is equal and opposite to the rotational speed of the drill string. Such contra-rotation is caused by an electric servo motor which drives an eccentric that engages a spigot or faucet on a bit drive shaft extension. The servo motor and a control unit therefor appear to be powered by a battery pack which includes sensors that are alleged to sense instantaneous azimuth or direction of a hypothetical reference radius of the tool. However, due to the electronic sophistication of this device it is unlikely to survive for very long in a hostile downhole drilling environment, so that its reliability may leave much to be desired.
An object of the present invention is to provide new and improved drilling tools and methods where the drilling of a directional wellbore can be accomplished while the drill string is being rotated.
Another object of the present invention is to provide new and improved drilling tools and methods for drilling a directional wellbore whereon the bit can be steered to stay on a desired course.
Still another object of the present invention is to provide new and improved drilling tools and methods where the rotation axis of the bit, or toolface, always points in one direction in space irrespective of the rotation of the drill string.